Revert "Consider methods added via attr_* no commands"

Althugh there is an issue in here, the current implementation is
very brittle as looking into the stacktrace is not guaranteed to
stay the same across different ruby versions and ruby implementations.

This reverts commit fb149e3dd8b71124d194cee81e5f5f1f788be6c5.
Allow calling private methods on template names

This is an expected feature from Thor. When rendering files,
Thor behaves like Sinatra. It works as if the file was embedded
in the current Thor class. Therefore, restricting the filename
to be just public methods is a wrong limitation that does not
fit with how Thor is meant to be used.
Merge pull request #316 from josevalim/thor-reverts

Revert regressions found in 0.18.0